Chemistry plays an important role in the interstellar medium (ISM), regulating the heating and cooling of the gas and determining abundances of molecular species that trace gas properties in observations. Although solving the time-dependent equations is necessary for accurate abundances and temperature in the dynamic ISM, a full chemical network is too computationally expensive to incorporate in numerical simulations. In this paper, we propose a new simplified chemical network for hydrogen and carbon chemistry in the atomic and molecular ISM. We compare results from our chemical network in detail with results from a full photodissociation region (PDR) code, and also with the Nelson & Langer (1999) (NL99) network previously adopted in the simulation literature. We show that our chemical network gives similar results to the PDR code in the equilibrium abundances of all species over a wide range of densities, temperature, and metallicities, whereas the NL99 network shows significant disagreement. Applying our network in 1D models, we find that the CO-dominated regime delimits the coldest gas and that the corresponding temperature tracks the cosmic-ray ionization rate in molecular clouds. We provide a simple fit for the locus of CO-dominated regions as a function of gas density and column. We also compare with observations of diffuse and translucent clouds. We find that the CO, CH x and OH x abundances are consistent with equilibrium predictions for densities n = 100−1000 cm −3 , but the predicted equilibrium C abundance is higher than observations, signaling the potential importance of non-equilibrium/dynamical effects. arXiv:1610.09023v4 [astro-ph.GA] 4 Apr 2019 PhotochemistryThe photodissociation reactions induced by FUV depend on the radiation field strength. We assume that the incident radiation field scales with the standard interstellar radiation field determined by Draine (1978), using the parameter χ as the field strength relative to J FUV = 2.7 × 10 −3 erg cm −2 s −1 (G 0 = 1.7 in Habing (1968) units). The photochemistry reaction rates appropriate for this radiation field are listed in Table 2.In optically thick regions, the radiation is attenuated by dust and by molecular line shielding. In a planeparallel slab geometry with beamed incident radiation equations, and found the results are the same. When calculating the abundance of H, C, O, and e from conservation laws, we assume all CHx and OHx are in the form of CH and OH. Because the abundance of CHx and OHx is usually very small compared to the hydrogen in H or H 2 , carbon in C, C + or CO, oxygen in O, and electrons provided by H + or C + , this assumption has no significant effect on the chemical network.

is the most widely used observational tracer of molecular gas. The observable luminosity is translated to mass via a conversion factor, , which is a source of uncertainty and bias. Despite variations in , the empirically determined solar neighborhood value is often applied across different galactic environments. To improve understanding of , we employ 3D magnetohydrodynamics simulations of the interstellar medium (ISM) in galactic disks with a large range of gas surface densities, allowing for varying metallicity, far-ultraviolet (FUV) radiation, and cosmic-ray ionization rate (CRIR). With the TIGRESS simulation framework we model the three-phase ISM with self-consistent star formation and feedback, and post-process outputs with chemistry and radiation transfer to generate synthetic CO (1–0) and (2–1) maps. Our models reproduce the observed CO excitation temperatures, line widths, and line ratios in nearby disk galaxies. decreases with increasing metallicity, with a power-law slope of −0.8 for the (1–0) line and −0.5 for the (2–1) line. also decreases at higher CRIR and is insensitive to the FUV radiation. As density increases, first decreases owing to increasing excitation temperature and then increases when the emission is fully saturated. We provide fits between and observable quantities such as the line ratio, peak antenna temperature, and line brightness, which probe local gas conditions. These fits, which allow for varying beam size, may be used in observations to calibrate out systematic biases. We also provide estimates of the CO-dark fraction at different gas surface densities, observational sensitivities, and beam sizes.

CO(J = 1 − 0) line emission is a widely used observational tracer of molecular gas, rendering essential the X CO factor, which is applied to convert CO luminosity to H 2 mass. We use numerical simulations to study how X CO depends on numerical resolution, non-steady-state chemistry, physical environment, and observational beam size. Our study employs 3D magnetohydrodynamics (MHD) simulations of galactic disks with solar neighborhood conditions, where star formation and the three-phase interstellar medium (ISM) are self-consistently regulated by gravity and stellar feedback. Synthetic CO maps are obtained by post-processing the MHD simulations with chemistry and radiation transfer. We find that CO is only an approximate tracer of H 2 . On parsec scales, W CO is more fundamentally a measure of mass-weighted volume density, rather than H 2 column density. Nevertheless, X CO = 0.7−1.0×10 20 cm −2 K −1 km −1 s consistent with observations, insensitive to the evolutionary ISM state or radiation field strength if steady-state chemistry is assumed. Due to non-steady-state chemistry, younger molecular clouds have slightly lower X CO and flatter profiles of X CO versus extinction than older ones. The CO-dark H 2 fraction is 26 − 79%, anti-correlated with the average extinction. As the observational beam size increases from 1 pc to 100 pc, X CO increases by a factor of ∼ 2. Under solar neighborhood conditions, X CO in molecular clouds is converged at a numerical resolution of 2 pc. However, the total CO abundance and luminosity are not converged even at the numerical resolution of 1 pc. Our simulations successfully reproduce the observed variations of X CO on parsec scales, as well as the dependence of X CO on extinction and the CO excitation temperature.

We investigate prestellar core formation and accretion based on three-dimensional hydrodynamic simulations. Our simulations represent local ∼ 1pc regions within giant molecular clouds where a supersonic turbulent flow converges, triggering star formation in the post-shock layer. We include turbulence and self-gravity, applying sink particle techniques, and explore a range of inflow Mach number M = 2 − 16. Two sets of cores are identified and compared: t 1 -cores are identified of a time snapshot in each simulation, representing dense structures in a single cloud map; t coll -cores are identified at their individual time of collapse, representing the initial mass reservoir for accretion. We find that cores and filaments form and evolve at the same time. At the stage of core collapse, there is a well-defined, converged characteristic mass for isothermal fragmentation that is comparable to the critical Bonner-Ebert mass at the post-shock pressure. The core mass functions (CMFs) of t coll -cores show a deficit of high-mass cores ( 7M ) compared to the observed stellar initial mass function (IMF). However, the CMFs of t 1 -cores are similar to the observed CMFs and include many low-mass cores that are gravitationally stable. The difference between t 1 -cores and t coll -cores suggests that the full sample from observed CMFs may not evolve into protostars. Individual sink particles accrete at a roughly constant rate throughout the simulations, gaining one t coll -core mass per free-fall time even after the initial mass reservoir is accreted. High-mass sinks gain proportionally more mass at late times than low-mass sinks. There are outbursts in accretion rates, resulting from clumpy density structures falling into the sinks.